Adsorption / Desorption Behaviors of Acetone over Micro-/ Mesoporous SBA-16 Silicas Prepared from Rice Husk Agricultural Waste Wanting

The preparation of micro-/mesoporous SBA-16 adsorbent by rice husk derived sodium silicate as the silica source and its acetone adsorption/desorption behaviors were reported for the first time. The pore structural properties of waste-derived SBA-16 materials were controlled and optimized by adjusting the surfactant/silica molar ratios from 0.002 to 0.01 for achieving the best acetone adsorption performance. And the relationship between structural properties and acetone adsorption performances of SBA-16 adsorbents was investigated. The resultant waste-derived SBA-16 materials, denoted as RSBA-16, had specific surface areas of 575–1001 m g, pore sizes of 3.3–3.7 nm and pore volumes of 0.41–0.72 cm g. Among the studied adsorbents, RSBA-16(0.004) exhibited the highest saturated acetone adsorption capacity of 179 mg g. It also showed excellent regeneration stability during 20 consecutive cycles. The results indicated that specific surface areas in both microand meso-pore ranges were the main factor that determined the superiority of acetone adsorption capacity of RSBA-16(0.004) over other adsorbents such as mesoporous RMCM-41 and micro-/mesoporous RSBA-15. The acetone adsorption experiments at 500–5000 ppmv demonstrated that the adsorption isotherm was well fitted with the Langmuir model, thus the adsorption of acetone on RSBA-16 might be a monolayer physisorption process. The results demonstrated the high adsorption capacity and cyclic stability of waste-derived RSBA-16(0.004) can be considered as a potential adsorbent for VOCs removals.


INTRODUCTION
The control of volatile organic compounds (VOCs) emitted from industrial processes is an important issue because of their toxicity, odor and as O 3 precursors (Hseu et al., 2013;Wu et al., 2015).And adsorption is a popular VOCs control process due to its simple operation with efficient recovery of most VOCs (Jo and Chun, 2014;Bernabe et al., 2015), with activated carbons (AC) as the most commonly employed VOCs adsorbents.However, the use of AC involves a number of problems such as its flammable property and regeneration difficulty (Fayaz et al., 2015;Mohammed et al., 2015;Niknaddaf et al., 2016).
Ordered mesoporous silicas such as M41S and SBA-type materials have recently attracted enormous attention as alternative VOCs adsorbents because of their uniform and tailorable pore system as well as high specific surface area (Gibson, 2014).In this aspect, research studies on adsorptive removal of various VOCs over MCM-41, SBA-15, and transfer of molecules within the pore channels (Rivera-Muñoz and Huirache-Acuña, 2010;Li et al., 2016).Moreover, there are also a large number of micropores across the silica walls in SBA-16 (Van Der Voort et al., 2002).These unique properties together with its good thermal stability of the thick wall and economical synthesis with inexpensive pore templates may make SBA-16 an attractive adsorbent for the environmental protection applications (Wei et al., 2008;Chaudhuri et al., 2016).Nevertheless, unlike the hexagonal form of SBA-15, the cubic form of SBA-16 mesoporous silica received much less attention.To the best of authors' knowledge, the research on adsorption of VOCs using SBA-16 made from either pure chemicals or waste resources has not been reported.
Using cheap and abundant silica sources instead of expensive ones is important to the production of silica based adsorbents at an industrial level (Chandrasekar et al., 2008;Gérardin et al., 2013).From this perspective, the recycling of silicon-rich solid wastes as alternative precursors will be highly practical and attractive (Ma and Ruan, 2013;Wang et al., 2013;Kim et al., 2015).In this work, SBA-16 materials are synthesized from the silica extracted from rice husk ash and applied for VOCs adsorption.Acetone is chosen as the representative compound because of its large usage in plastics, pharmaceutical, and semiconductor industries.The pore structure and BET characteristics of waste-derived SBA-16 materials were controlled and optimized by adjusting the surfactant/silica molar ratio for achieving the best adsorption performance of acetone, and the relationship between pore structure and adsorption performance was demonstrated.The performance of waste-derived SBA-16 adsorbents was also compared to that of SBA-16 made from pure chemical precursors as well as MCM-41, and SBA-15 manufactured by using rice husk derived sodium silicate as the silica precursors.

Extraction of Silicate Supernatant
Rice husk ash (RHA) was obtained by heating rice husk at 400°C.The RHA was then mixed with aqueous 6M NaOH solution at 70°C for 12 h, and the resulting mixture was centrifuged to separate the silicate supernatant and sediment.The silicate supernatant was then utilized as the silica precursor for the synthesis of SBA-16 materials and named as RSBA-16 hereafter.

Synthesis of Adsorbents
The RSBA-16 material was synthesized using a tri-block copolymer, EO 106 PO 70 EO 106 (Pluronic F127, BASF) as the template and RHA-derived silicate solution as the silica source.The molar composition of the gel mixture was 1 SiO 2 : 0.002-0.010F127: 286 H 2 O.For a typical synthesis, a given amount of F127 was dissolved in 20 g of DI water.Meanwhile, 82 mL of silicate solution was added to 70 mL of 6 M H 2 SO 4 to achieve the pH value of 2. Subsequently, 6 M NaOH was added to the above silicate mixture to bring up the pH to 5 for accelerating the hydrolytic condensation reaction.After stirring, the surfactant solution was added slowly into the above mixture and the combined mixture was stirred for 15 min.The resulting gel mixture was transferred into a Teflon coated autoclave and kept in an oven at 100°C for 24 h.After cooling to room temperature, the resultant solid was recovered by filtration, washed with DI water and dried in an oven at 60°C for 24 h.Finally, the organic template was removed by using a muffle furnace in air at 500°C for 6 h.The RSBA-16 adsorbents synthesized from RHA at different Surf/Si molar ratios were named as RSBA-16(0.002),RSBA-16(0.004),RSBA-16(0.006),RSBA-16(0.008),and RSBA-16 (0.01).
For comparison purpose, the SBA-16 material was also prepared by using commercial chemical reagent of tetraethylorthosilicate (TEOS) as the silica precursor, and was denoted as SBA-16(TEOS).The molar composition of gel mixture was 1 SiO 2 : 0.004 F127: 3 NaCl: 110 H 2 O: 3.4 HCl.Detailed synthetic information for the synthesis of SBA-16(TEOS) can be referred to the literature (Wei et al., 2008).
The synthesis of MCM-41 and SBA-15 from waste RHA was performed as well.The waste-derived MCM-41 was obtained by using RHA-derived silicate solution and cetyltrimethylammonium bromide (CTAB).The molar composition of the gel mixture was 1SiO 2 :0.2CTAB:120H 2 O: 0.89H 2 SO 4 .In a typical procedure, 89 mL of waste silicate solution was firstly acidified by adding approximately 40 mL of 2 M H 2 SO 4 to bring down the pH to 10.5 with constant stirring to form a gel.After stirring, 7.28 g of CTAB (dissolved in 25 mL of DI water) was added drop by drop into the above mixture and the combined mixture was stirred for three additional hours.The resulting gel mixture was transferred into a Teflon coated autoclave and kept in an oven at 145°C for 36 h.After cooling to room temperature, the resultant solid was recovered by filtration, washed with DI water and dried in an oven at 110°C for 8 h.Finally, the organic template was removed in a muffle furnace (in air) at 550°C for 6 h.
The preparation of RHA-derived SBA-15 was similar to that of RSBA-16, except that EO 20 -PO 70 -EO 20 (Pluronic P123, BASF) was used as the template.The organic P123 template was removed in a muffle furnace (in air) at 500°C for 6 h.The recycled MCM-41 and SBA-15 materials using RHA-derived silicate solution as the silica source was denoted as RMCM-41 and RSBA-15, respectively.

Characterization
The specific surface area and total pore volume of the samples were measured by nitrogen adsorption-desorption isotherms at -196°C using a surface area analyzer (Micromeritics, ASAP 2000).All the samples were degassed for 6 h at 300°C under vacuum (10 -6 mbar) prior to the adsorption experiments.The specific surface areas of adsorbents were calculated by Brunauer-Emmett-Teller (BET) method in the relative pressure range P/P 0 = 0.05-0.3.The average pore sizes (D p ) of the adsorbents were determined by the Barrett-Joyner-Halenda (BJH) method, and their total pore volumes were obtained by the nitrogen amount adsorbed at P/Po = 0.99.The pore size distribution of MCM-41 was obtained from the analysis of the desorption branch of the isotherm by the BJH method, while nonlocal density functional theory (NLDFT) method was used to determine the pore size distributions of both RSBA-15 and RSBA-16 samples (Liu et al., 2016).The micropore size distributions of RSBA-15 and RSBA-16(0.004)were determined by the Horvath-Kawazoe model, and the micropore surface area and volume were calculated by the t-plot method.
The powder X-ray diffraction (XRD) analyses were made to reveal the crystalline structure of mesoporous materials, and the diffractograms of the mesoporous samples were recorded in the 2θ range of 0.5-4° in steps of 0.1 degree with a count time of 60 s at each point.Acetone temperatureprogrammed desorption (Acetone-TPD) was performed with an AutoChem II 2920 apparatus.Prior to the acetone-TPD experiments, the samples were preheated in air at 200°C for 2 h and cooled to 25°C.Then the samples were exposed to a flow of 500 ppmv acetone/He at 50°C for 1 h and then followed by argon purging for 1 h.Finally, the temperature was raised to 900°C in an Ar flow at a heating rate of 10 °C min -1 .

Acetone Adsorption Tests
Acetone adsorption experiments were performed with a lab-built packed column adsorber.The adsorbents were pelletized and sieved to size between 16-30 mesh (0.60-1.19 mm), and a total of 500 mg adsorbent was packed into an adsorption column with internal diameter of 0.7 cm.The adsorbents were pretreated before adsorption under nitrogen flow of 0.1 L min -1 at 200°C for 1 h.They were then cooled to 25°C for adsorption tests.Under a typical adsorption test, the acetone concentration was controlled at 1000 ppmv.But for obtaining the adsorption isotherm the concentrations of acetone were then varied in the range of 500-5000 ppmv.The packed column tests were performed with a total flow rate of 500 cm 3 min -1 .The concentration of acetone was continuously measured by a gas chromatograph (GC 7890, Agilent) equipped with a flame ionization detector (FID).The breakthrough time was defined at which the outlet concentration was about 10% of the inlet concentration.This corresponded to 90% acetone removal efficiency, and the acetone adsorption capacity at which is referred as the working capacity.
The Langmuir, Freundlich and Dubinin-Radushkevich equations were applied to study the acetone adsorption isotherms obtained at various inlet acetone concentrations.The Langmuir equation assumes monolayer adsorption onto a surface with a finite number of identical sits, and is represented as follows (Langmuir, 1916(Langmuir, , 1918)): where C e (ppmv) and q e (mg g -1 ) are the gas phase concentration and solid phase adsorption quantity of acetone at equilibrium, respectively; K a is the Langmuir isotherm constant; q m (mg g -1 ) is the single layer acetone adsorption capacity.The Freundlich equation is an empirical model that assumes the equilibrium adsorption capacity is linear with a change in adsorbate concentration on a log-log scale at a given temperature (Ruthven, 1984): where C e (ppmv) and q e (mg g -1 ) are the gas phase concentration and solid phase adsorption quantity of acetone at equilibrium; K F is the Freundlich constant; and n f is the heterogeneity factor.
The Dubinin-Radushkevich (DR) isotherm model is developed to determine the physisorption in microporous adsorbents, following a pore filling mechanism.The Dubinin-Radushkevich equation is as follows (Dubinin, 1989): where C e (ppmv) and q e (mg g -1 ) are the gas phase concentration and solid phase adsorption quantity of acetone at equilibrium; q s (mg g -1 ) is the theoretical isotherm saturation capacity; K DR (mol 2 kJ -2 ) is a constant related to the mean free energy of adsorption; ε is the Dubinin-Radushkevich isotherm constant; R (J mol -1 K -1 ) is the gas constant; and T (K) is the absolute temperature.
For understanding the stability of acetone adsorption/ desorption process using the materials, a total of 20 adsorption/desorption cycles were performed in order to investigate the reversibility of acetone adsorption/desorption during the regeneration process.The adsorbents were activated by heating to 200°C for 1 h under a 0.1 L min -1 nitrogen flow.After cooling to 25°C, 1000 ppmv acetone was introduced until 90% acetone removal efficiency was reached.Subsequently, the acetone flow was then disconnected and the nitrogen flow at 0.1 L min -1 flow rate was introduced.The adsorbent was heated to 200 C and kept constant for 30 min to desorb acetone from the adsorbent.

Characterization of RSBA-16
Fig. 1 shows the powder XRD patterns of RSBA-16 samples prepared under various Surf/Si molar ratios.For the RSBA-16 samples, all of the XRD patterns showed one distinct diffraction reflection at 2θ value of 0.73-0.83°,which could be indexed as (1 1 0) reflection corresponding to a cubic Im3m structure (Hwang et al., 2004;Wei et al., 2008).It is found that the crystallinity of RSBA-16 samples was very sensitive to the Surf/Si molar ratio of the initial gel mixture.When the Surf/Si molar ratio of the initial gel was below 0.004 or above 0.008, one can see that crystallinity of RSBA-16 materials tended to decrease, suggesting the collapse of long range crystallographic order (Cheng et al., 2003;Van Der Voort et al., 2002).In addition, it is observed that the intensity of (1 1 0) reflection of RSBA-16(0.004)was similar to that of SBA-16(TEOS), which indicated the structural quality of RSBA-16(0.004)was as good as the SBA-16(TEOS) sample synthesized from TEOS.On the basis of the above results, it is therefore concluded that the excessive or insufficient surfactant concentration in the precursor mixture seemed unfavorable for establishing cubic architecture of RSBA-16.
The N 2 adsorption/desorption isotherms of all RSBA-16 samples are depicted in Fig. 2, where all samples displayed type IV isotherm of typical mesoporous materials according to the IUPAC classification, with a sharp capillary condensation step at intermediate P/Po of 0.4-0.6 (Zhao et al., 1998).Furthermore, an obvious H2-typed hysteresis loop was observed for all RSBA-16 samples, which revealed the evidence of cage-like mesostructure with interconnectivity in a 3D pore system of SBA-16 materials (Gobin et al., 2007).Also, it is noted that RSBA-16 samples showed a steep nitrogen uptake at very low relative pressure (P/Po = 0.02), which was related to the filling of micropores (Meynen et al., 2009).The above results revealed that RSBA-16 samples exhibited a biporous system consisting of cubic mesopore channels that were connected by micropores (Kleitz et al., 2003;Almeida et al., 2012).
The BET specific surface area (S BET ), pore diameter (D p ) and total pore volume (V total ) of RSBA-16 are summarized in Table 1.It is observed both S BET and V total had maxima of 1001 m 2 g -1 and 0.72 cm 3 g -1 at a Surf/Si ratio of 0.004.And one can also see that the amount of micropores (V micro ) decreased continuously as a function of increasing Surf/Si ratio.It was reported that for the synthesis of SBA-16 materials at low Surf/Si ratios, microporous plugs were formed inside the 3D pore system which increased the amount of micropores in the system (Kim et al., 2004).This is because that some of the triblock copolymers might not be involved in the actual templating of the mesopores, but still acted as templates for the formation of microporosity.Furthermore, it was noted that the surface area of RSBA-16(0.004)(1001 m 2 g -1 ) was almost the same as that of SBA-16(TEOS) (1004 m 2 g -1 ) prepared by using TEOS pure chemical as the silica precursor, revealing the high quality of the obtained RSBA-16(0.004).

Adsorption Performance of Acetone over RSBA-16
The breakthrough curves of 1000 ppmv acetone adsorption at 25°C over different RSBA-16 adsorbents are shown in Fig. 3.For comparison purpose, mesoporous material of RMCM-41 as well as micro-/mesoporous materials of RSBA-15 and SBA-16(TEOS) were also tested for their adsorption performance.And their structural properties are summarized in Table 1.One can see in Table 1 that these three materials have very high specific surface areas of over 1000 m 2 g -1 .But the RSBA-15 has a relatively low value of specific surface area in the micropore range (119 m 2 g -1 ) as compared to those of the RSBA-16(0.004)(335 m 2 g -1 ) and SBA-16(TEOS) (311 m 2 g -1 ).Fig. 3 shows that all adsorbents had initial adsorption efficiencies of near 100%.And it is noticeable that the breakthrough time and the saturated time (C/C o = 1.0) of the RSBA-16(0.004)were the longest among all tested adsorbents.This revealed that RSBA-16(0.004)had the highest acetone capacity among all tested adsorbents.
Table 1 summarizes the saturated acetone adsorption capacities of all adsorbents, and RSBA-16(0.004)has the highest acetone adsorption capacity of 179 mg g-adsorbent -1 among all tested adsorbents.To better understand the equilibrium acetone adsorption characteristics of RSBA-16 materials, the RSBA-16(0.004)sample was selected as the representative adsorbent because of its largest adsorption capacity.The experimental data were substituted into the Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherm models listed in Eqs. ( 1), ( 2) and (3), and the regression results are shown in Fig. 4. The equilibrium adsorption capacities (q e ) increased first and then remained  f T-plot micropore volume.g The relative prices of chemicals were calculated based on the ratios of the purchase price of chemicals used for manufacturing the adsorbents to that for the manufacture of RSBA-16(0.004).almost the same when further increasing the acetone concentration (C e ).The calculated correlation coefficients (R 2 ) between these isotherm equations and experimental data were 0.999, 0.850 and 0.937, respectively, for Langmuir, Freundlich and D-R isotherm models.These observations infer that monolayer physisorption could be the main process since the Langmuir adsorption isotherm model showed the best agreement with the experimental data.
It is clear from Fig. 4 that Langmuir equation is the best isotherm model to describe the experimental observation.The Langmuir model assumes monolayer coverage of adsorbate on the surface of adsorbent, thus it is expected that the acetone adsorption capacity should be linearly correlated to the specific surface area of the adsorbent.This is demonstrated by Fig. 5 for the linear increase of acetone adsorption capacity with the increase of specific surface area.The linear regression coefficient (R 2 ) between the acetone adsorption capacity and the specific surface area of RSBA-16 adsorbents was 0.96.A similar observation has been found over SBA-15 materials for the adsorption of naphthalene (Liu et al., 2016).
As can be seen in Table 1, the acetone capacity of RSBA-16(0.004)was similar to that of SBA-16(TEOS) made from commercial chemicals.This is expected since they both had very similar pore textural properties.In addition, although these three materials of RSBA-16(0.004),RMCM-41 and RSBA-15 had similar specific surface areas of around 1000 m 2 g -1 , the adsorbent of RSBA-16(0.004)showed a better adsorption capacity than the other two adsorbents.This might be due to that the presence of micropores in the mesoporous silica materials is beneficial for enhancing the adsorption performances of VOCs.The pore size closed to the molecular size of gaseous molecules might have a molecular sieving effect, which acted as a trap for retaining the gaseous molecules within the pores (Kosuge et al., 2007;Zhang et al., 2012).Fig. 6(a) depicts the micropore size distributions of RSBA-15 and RSBA-16(0.004).Both samples showed similar micropore size distribution in the range of 0.4-0.9nm with a mode peak at ca. 0.55 nm, which was close to the molecular size of acetone (0.43 nm).And RSBA-16(0.004)exhibited a higher peak intensity (dV/dD) on this micropore range than that of the RSBA-15.Furthermore, Fig. 6(b) showed that RSBA-16(0.004)also displayed a higher peak intensity on small mesopores (D p < 3 nm).The observations from Figs. 6(a) and Fig. 6(b) are consistent with the higher value of micropore surface area of RSBA-16(0.004)than that of RSBA-15 as shown in Table 1.This also helps to explain that the observed monolayer adsorption in Figs. 4 and 5 since RSBA-16 adsorbent has a significant portion in micropore and small mesopore ranges.This is also the main reason for the superior adsorption performance of RSBA-16(0.004)as compared to that of the RMCM-41 which has only mesopores.Accordingly, one may conclude that specific surface areas in both micro-and meso-pore ranges were the main factor that determined the superiority of acetone adsorption capacity of RSBA-16(0.004)adsorbent over other adsorbents such as mesoporous RMCM-41 and micro-/mesoporous RSBA-15.
For the practical application, the adsorbent is replaced or regenerated at the break point to maintain the designed removal efficiency (e.g., 90% removal).One can see in Table 1 that the break point (at 90% acetone removal efficiency) adsorption capacity of RSBA-16(0.004)sample was 152 mg g -1 .This was much higher than those of RMCM-41 (79 mg/g) and RSBA-15 (116 mg g -1 ) under the same conditions.Table 2 compares the acetone adsorption capacities of this study with others available in the literature.It is observed that the RSBA-16(0.004)developed in this study had the highest acetone adsorption capacity among all adsorbents under comparable conditions except the coconut based activated carbon.The adsorption capacity of coconut-derived activated carbon (117 mg g -1 at 20°C, 200 ppmv inlet condition) had a similar adsorption capacity to that of RSBA-16(0.004)(115 mg g -1 at 25°C, 200 ppmv inlet condition).However, the flammable nature of the carbon-based sorbent might limit its potential use for cyclic VOCs adsorption-desorption applications.

Cyclic Acetone Adsorption/Desorption Tests
The acetone-TPD measurement was performed to understand the desorption behaviors of RSBA-16(0.004),RMCM-41, and RSBA-15 samples, with results shown in Fig. 7.It has been reported that the desorption performance of adsorbent is mainly associated with the interaction between adsorbent surface and adsorbate molecules (Zhang et al., 2012).All mesoporous silica adsorbents showed a sharp desorption peak in the range of 100-200°C.Compared to RMCM-41 and RSBA-15 samples, the TPD profile of RSBA-16(0.004)showed a slight shift toward higher temperature, suggesting a stronger interaction between acetone molecules and RSBA-16(0.004).This observation could be due to the difference in the micro-/mesoporous structure between these three adsorbents as discussed previous by Fig. 6.The presence of micropores (0.4-0.9 nm) and relatively small mesopores (D p < 3 nm) may result in strong interaction between the acetone molecule and the pore surface of RSBA-16(0.004).
Temperature swing adsorption (TSA) process has been widely adopted for the adsorptive gas separation in practical application (Gabruś et al., 2015).As evidenced from the acetone-TPD results, most of acetone molecules can be effectively desorbed from the adsorbent surfaces of RMCM-41, RSBA-15 and RSBA-16(0.004)at 200°C.Therefore, the following cyclic adsorption/desorption tests were conducted using 200°C as the desorption temperature, and 25°C as the adsorption temperature.Fig. 8 displays the working capacity and adsorption index of  during 20 cycles of adsorption/desorption operation.The adsorption index was calculated as the percentage ratio of the working capacities of the regenerated adsorbents to that of the fresh adsorbent, thus 100% adsorption index implies that the adsorbent was not deteriorated at all.
It can be seen in Fig. 8 that the acetone working capacity for RMCM-41, RSBA-15, and RSBA-16(0.004)decreased from 79, 116 and 152 mg g -1 for the fresh adsorbents to 75, 106 and 140 mg g -1 , respectively, for adsorbents after the first cycle of operation.Then the adsorption capacities were quite stable during the following 19 cycles of operation for all materials.The average adsorption indexes of RMCM-41, RSBA-15 and RSBA-16(0.004)adsorbents during the 20 regeneration cycles were 91.7 ± 2.4%, 91.4 ± 1.5% and 90.5 ± 1.3%, respectively.The insignificant difference on the cyclic adsorption/desorption performance over these three samples could be attributed to the intrinsic nature of acetone, which is a polar compound with small molecular size and high vapor pressure, making it to be easily desorbed from all three materials.As a result, RSBA-16(0.004)still retained the highest adsorption capacity of 137 mg g -1 as compared to those of RMCM-41 (72 mg g -1 ) and RSBA-15 (106 mg g -1 ) adsorbents after 20 cycles.This result indicates that RSBA-16(0.004)can be employed as a stable acetone adsorbent through cyclic TSA operation.
Besides having high adsorption performance and regeneration stability, the production cost is also one of the most important considerations for industrial applications.It  can be seen in Table 1 that the chemical cost of RSBA-16(0.004) is only two-third to that of SBA-16(TEOS).And when considering the need of treatment and disposal of rice husk waste, then the cost of RSBA-16 materials can be decreased further.On the other hand, even though the chemical cost of RSBA-16(0.004) is slightly higher than that of RSBA-15, the acetone adsorption capacity (179 mg g -1 ) of RSBA-16(0.004) is much higher than that of RSBA-15 (144 mg g -1 ).Therefore, for obtaining the same acetone adsorption capacity, the quantity of RSBA-16(0.004)adsorbent can be reduced in comparison with that of RSBA-15.This would provide a high advantage for practical adsorption application using RSBA-16(0.004)since it can reduce the adsorber volume of VOCs adsorption.And RSBA-16(0.004)also showed much lower chemical cost and much higher adsorption capacity than those of RMCM-41.Accordingly, RSBA-16(0.004) is a better adsorbent for VOCs removals owing to its superior adsorption capability and higher adsorption capacity/price value.
For the synthesis of SBA-16 materials, the use of the commercial silicate source (e.g., TEOS) accounts for a significant portion of the manufacturing cost, since the other reagent (e.g., the structure directing template of F127) can be easily recycled using the solvent extraction approach under mild conditions (Jia et al., 2013;Ng et al., 2013).Therefore the use of rice husk agricultural waste instead of commercial silica source as a cheap and abundant silica precursor appears to be highly beneficial for synthesizing RSBA-16 materials at an industrial level.

CONCLUSIONS
In this work, the adsorption/desorption behaviors of acetone on SBA-16 adsorbents prepared by using silicate precursor from rice husk waste has been demonstrated for the first time.The results revealed that specific surface areas in both micro-and meso-pore ranges were the main parameter affecting the acetone adsorption performance of rice husk ash-derived SBA-16 adsorbents.And it was demonstrated that rice husk ash-derived SBA-16(0.004)had a very similar pore structure and acetone capacity to those of the SBA-16 made from pure chemicals.This reveals the high potential of using rice husk waste to manufacture SBA-16 material of the same quality as that made with pure chemicals.Compared to rice husk ash-derived MCM-41 and SBA-15 adsorbents, rice husk ash-derived SBA-16(0.004)showed important advantages of high adsorption capacity and good cyclic adsorption/desorption stability.Moreover, to adsorb the same quantity of acetone, the amount of rice husk ash-derived SBA-16(0.004)was also lower than that of rice husk ash-derived SBA-15.Consequently, less space was required when rice husk ash-derived SBA-16(0.004)was used as an adsorbent.Future work is needed to test on the adsorption of different adsorbates such as toluene and 1,2,4-trimethylbenzene (TMB) so that the properties and roles of SBA-16 micro-/mesoporous adsorbents can be further clarified when applied to the VOCs adsorption processes.

Table 1 .
Structural parameters and adsorption performance of porous silica materials.
a Sample name with "R" indicated that the material was prepared using rice husk derived precursor.b BET surface area.c Micropore surface area.d Average pore diameter calculated by BJH theory.e Total pore volume.

Table 2 .
Comparison of acetone adsorption capacity of various adsorbents tested by this study and literature.